Copper complexes aziridination catalysts

The reaction used to test these solid catalysts was the aziridination of styrene with AT-tosyliminophenyliodinane (Phi = NTos) (Scheme 10). In most cases, enantioselectivities were low or moderate (up to 60% ee). The loss of enantioselectivity on changing from ligand 11a to ligand 12 was attributed to the fact that ligand 12 is too big for the copper complex to be accommodated into the zeolite supercages. Further studies carried out with hgands 11a and 11b [62] demonstrated that the reaction is more enantioselective with the supported catalyst (82% ee with 11a and 77% ee with 11b) than in solution (54% ee with 11a and 31% ee with 11b). This trend supports the confinement effect of the zeolite structure on the stereoselectivity of the reaction. 

Bis(oxazoline)-copper complexes 158 have been used by Evans group as chiral catalysts for the enantioselective aziridination of olefins.116 Aryl-substituted olefins have been found to be particularly suitable substrates, which can be efficiently converted to A-tosylaziridines with ee of up to 97% (R = Ph... 

Copper complexes catalyze formally related aziridination of olefins with ]7V-(p-toluenesulfonyl)imino]phenyliodinane, a nitrene precursor (219b). As exemplified in Scheme 98, catalysts formed from Cu(I) tri-flate and optically active bis(oxazolines) effect enantioselective reaction of styrene (Scheme 98) (218b, 219a). 

In 1991, Evans et al. reported that cationic Cu(I) ions catalyzed the nitrene-transfer reaction smoothly (Scheme 6B.29) [74]. Since then, many studies on asymmetric aziridination have been carried out with chiral copper(I) complexes as catalysts. 

Lowenthal and Masamune also reported that the copper complex bearing a bisoxazoline ligand 30 was an effective catalyst for aziridination of styrene (88% ee) (Scheme 6B.31) [76], However, Evans et al. later claimed that this result was not reproducible [75],... 

Attempts to aziridinate alkenes with iron catalysts in an asymmetric manner have met with only limited success to date [101], In an early report on the use of various chiral metal salen complexes, it was found that only the Mn complex catalyzed the reaction whereas all other metals investigated (Cr, Fe, Co, Ni etc.) gave only unwanted hydrolysis of the iminoiodinane to the corresponding sulfonamide and iodoben-zene [102], Later, Jacobsen and coworkers and Evans et al. achieved good results with chiral copper complexes [103]. 

Enantioselective Aziridination of Alkenes. Copper complexes with neutral methylenebis(oxazoline) ligands (1) and (2) have also been employed as enantioselective catalysts for the reaction of alkenes with (Al-tosylimino)phenyliodinane, leading to A-tosylaziridines. The best results have been reported for cinna-mate esters as substrates, using 5 mol % of catalyst prepared from CuOTf and the phenyl-substituted ligand (2) (eq 6). The highest enantiomeric excesses are obtained in benzene, whereas in more polar and Lewis basic solvents, such as acetonitrile, the selectiv-ities are markedly lower. The chemical yield can be substantially improved by addition of 4X molecular sieves. Both Cu - and Cu"-bisoxazoline complexes, prepared from Cu or Cu triflate, respectively, are active catalysts, giving similar results. In contrast to the Cu-catalyzed cyclopropanation reactions discussed above, in which only Cu complexes are catalytically active, here Cu complexes are postulated as the actual catalysts. ... 

An electrophilic metal-carbene complex intermediate has been proposed in the cyclopropanation reaction, whereas a paramagnetic copper nitrene species, which behaves as an electrophilic, nitrogen-centered radical, is proposed as the intermediate for the aziridination reaction.45 [Cu(Tp GF3 2)(C2H4)] is a good aziridination catalyst, readily converting a variety of olefins into the corresponding A-tosyl aziridines.46... 

The use of polypyrazolylborate copper(I) complexes as catalysts in the conversion of olefins into aziridines 01JOM(617-618)110. 

Jorgensen has recently reported similar enantioselective reactions between N-tosylimines 107 and trimethylsilyldiazomethane (TMSD) catalyzed by chiral Lewis acid complexes (Scheme 1.32) [57, 53]. The cis-aziridine could be obtained in 72% ee with use of a BINAP-copper(i) catalyst, but when a bisoxazoline-copper(i) complex was used the corresponding trans isomer was fonned in 69% ee but with very poor diastereoselectivity. 

A dinuclear iron(ll/Ill) complex bearing a hexadentate phenol ligand displayed moderate activity toward aziridination of alkenes with PhlNTs a large excess of alkene (2,000 equiv. vs PhlNTs) was required for good product yields (Scheme 22) [76]. It is noteworthy that complex 4 is active in the aziridination of aliphatic alkenes, affording higher product yields than copper (11) catalysts with tetradentate macrocyclic ligands [77]. 

Copper(I) complexes containing NHC-phenoxyimine 153 or NHC-phenoxyamine 154 were shown to be good catalyst systems for nitrene addition to alkenes 144 (Scheme 5.40) [45]. The catalyst systems showed to be highly efficient as only 1 mol% catalyst loading was required to afford aziridines 155 in moderate to good yields. 

Various approaches to epoxide also show promise for the preparation of chiral aziridines. Identification of the Cu(I) complex as the most effective catalyst for this process has raised the possibility that aziridination might share fundamental mechanistic features with olefin cyclopropanation.115 Similar to cyclo-propanation, in which the generally accepted mechanism involves a discrete Cu-carbenoid intermediate, copper-catalyzed aziridation might proceed via a discrete Cu-nitrenoid intermediate as well. 

The initial screen of potential catalysts by these workers revealed that several Lewis acids are capable of effecting nitrenoid transfer to alkenes. In particular, SmLOf-Bu, a species that is unlikely to participate in redox processes, was found to work well for 7ra s-p-methylstyrene aziridination. Although the generality of this catalyst fell far short of the copper system, it raises the intriguing possibility that the Cu(II) species formed in the aziridination acts at least in part as a Lewis acid. The considerable Lewis acidity of cationic Cu(II) complexes has since been extensively exploited (cf. Section V). 

In a study published concurrently with the Evans bis(oxazoline) results, Jacobsen and co-workers (82) demonstrated that diimine complexes of Cu(I) are effective catalysts for the asymmetric aziridination of cis alkenes, Eq. 66. These authors found that salen-Cu [salen = bis(salicylidene)ethylenediamine] complexes such as 88b Cu are ineffective in the aziridination reaction, in spite of the success of these ligands in oxo-transfer reactions. Alkylation of the aryloxides provided catalysts that exhibit good selectivities but no turnover. The optimal catalyst was found to involve ligands that were capable only of bidentate coordination to copper. 

The first catalytic, asymmetric aziridination of an alkene in good yield and high enantioselectivity was recently reported56. Thus styrene (63) was treated with [N-(p-toluenesulphonyl)imino]phenyliodinane (64) and an asymmetric copper catalyst to yield (/ )-Ar-(p-toluenesulphonyl)-2-phenylaziridine [(/ )-65] in 97% yield with an ee of 61%, the catalyst being the complex formed in situ in chloroform from the chiral bis[(5 ) 4-ferf-butyloxazoline] [(S,S)-66] and copper triflate (CuOTf)56, the reaction proceeding by way of a nitrene transfer57. 

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